Banana Beach security is only there in the evenings,but there are police that walk the beach all day long, at least they did at Christmas. I would not leave any money laying around on my lounge chair, but a towel and suntan lotion and drinks are fine.
I have also never heard of anyones lounge chair getting robbed ,all in all Bananas is very safe, the staff is always walking around the area,also. Plus there are people out there the whole time, that I am sure would not mind keeping an eye on your things.
Enjoy your trip!!! Banana Beach is the best.